Blue Phosphorescence and Hyperluminescence Generated from Imidazo[4,5‐b]pyridin‐2‐ylidene‐Based Iridium(III) Phosphors

Abstract Four isomeric, homoleptic iridium(III) metal complexes bearing 5‐(trifluoromethyl)imidazo[4,5‐b]pyridin‐2‐ylidene and 6‐(trifluoromethyl)imidazo[4,5‐b]pyridin‐2‐ylidene‐based cyclometalating chelates are successfully synthesized. The meridional isomers can be converted to facial isomers through acid induced isomerization. The m‐isomers display a relatively broadened and red‐shifted emission, while f‐isomers exhibit narrowed blue emission band, together with higher photoluminescent quantum yields and reduced radiative lifetime relative to the mer‐counterparts. Maximum external quantum efficiencies of 13.5% and 22.8% are achieved for the electrophosphorescent devices based on f‐tpb1 and m‐tpb1 as dopant emitter together with CIE coordinates of (0.15, 0.23) and (0.22, 0.45), respectively. By using f‐tpb1 as the sensitizing phosphor and t‐DABNA as thermally activated delayed fluorescence (TADF) terminal emitter, hyperluminescent OLEDs are successfully fabricated, giving high efficiency of 29.6%, full width at half maximum (FWHM) of 30 nm, and CIE coordinates of (0.13, 0.11), confirming the efficient Förster resonance energy transfer (FRET) process.

TD-DFT [2] using the B3LYP functional [3] with Gaussian 16 set of programs. [4] The polarizable continuum model (PCM) [5] was used to include solvent effects of toluene. The 6-31G(d,p) [6] basis set was used for light elements such as hydrogen, carbon, nitrogen, oxygen and fluorine, while the LANL2DZ [7] basis set with the Los Alamos National Laboratory (LANL) effective core potentials (ECPs) was used for iridium. The corresponding ground state (S 0 ) geometries were optimized based on the X-ray structural data of f-tpb1 and f-tpb2. The low-energy excited states were then calculated by using the TD-DFT method based on the optimized ground state structures. Avogadro software [8] was used to visualize the orbitals presented in this work. Orbital composition analysis was performed using the Hirshfeld method [9] to calculate the contribution of metal atom to each molecular orbital with the Multiwfn [10] software.
Relativistic TD-DFT calculations were further performed to obtain more detailed information on spin-mixed electron excitations (e.g., the oscillator strength and the radiative rate for S 0 → T 1 excitations) with ADF (2019 version) [11] using the B3LYP functional and Slater type TZP [12] basis sets. The COnductor-like Screening MOdel (COSMO) [13] is used to model solvent effects.
In these calculations, spin-orbit coupling (SOC) was added perturbatively [14] to one-component TDDFT [2a] utilizing the one-component zeroth order regular approximation (ZORA). [15] A total of 24 spin-mixed excitations were calculated. The radiative rate ( ) of an excited state was then calculated according to [16] = 2 2 / 3 (1) where is the excitation energy and is the corresponding oscillate strength for the electronic excitation from the ground state to excited state, and the equation is in atomic units with c representing the speed of light in vacuum. The three lowest energy states found in the SOC TDDFT calculations for the complexes considered here are derived from the three substrates in a triplet state with a small singlet admixture due to SOC. [17] There will be small energy differences between the three states because of their different admixtures with the singlet state, the so called zero-field splitting (ZFS). As the ZFS is small compared to k B T at room temperature, the radiative rate of the triplet state is then calculated as an average over the three substrates within the assumption of fast thermalization.
Device fabrication and measurement. Devices were fabricated on patterned ITO glass substrates were cleaned with tetrahydrofuran, deionized water and isopropanol in sequence, dried in an oven for at least 2 h, treated with UV-ozone for 10 min, and finally loaded into a deposition chamber with a basic pressure less than 5 × 10 −4 Pa. Deposition rates and thicknesses of all materials were monitored with oscillating quartz crystals. The deposited rates for organic materials, LiF, and Al were controlled at 1 -2, 0.1, and 7 Å s −1 , respectively.
The current density-voltage (J-V-L) characteristics of the devices were measured using a
After then, the mixture was poured into an ice-water mixture and neutralized to pH 8-9 by addition of conc. NH 4 OH (aq) and extracted with ethyl acetate. The combined organic layers were dried over anhydrous Na 2 SO 4 and concentrated to give a yellow solid (3.72 g). 1 H NMR (400 MHz, CDCl 3 ) δ 8.4 (d, 1H), 7.8 (d, 1H).
[b] Note that the calculated k r (at 0 K) is systematically smaller than the experimental k r (at RT), same as that found in other families of Ir(III) complexes. The former is intrinsic and independent from the non-radiative decay pathways, whereas the latter is highly dependent on the non-radiative decay pathways. The uncertainty of applied computational model and methods is also one of possible sources of deviation between calculated and experimental k r .